This patent relates to the field of rehabilitation of individuals suffering from a visual field loss. More particularly, this invention relates to an improved method of treating patients with visual field losses by using lens systems which incorporate a prism.
Individuals suffering from visual field loss experience severe confusion and disorientation. They find it extremely difficult to remove themselves from the roles of the disabled and lead a productive life. Their diagnosis is visual field neglect, a term used to indicate the inability to make judgments with regard to a localized area of their spacial world. In the case of hemianopsia, this localized area is to the right or left of the patient's spacial world; in altitudinal hemianopsia it is upward or downward. The site of insult along the optic pathway determines the nature of the visual field loss. The most common types of hemianopsia are usually caused by a head injury, cerebral vascular accident, or brain tumor requiring neurosurgery affecting the visual pathway. They can be congruous (identical in size, shape, position and density) or incongruous.
Compensatory techniques which have been considered for the management of visual field defects include the utilization of mirrors, partially reflecting mirrors, reverse telescopes and prisms. Such techniques are discussed in a recent article by Edward Goodlaw, 0.D., "Review of Low Vision Management of Visual Field Defects," Optometric Monthly, July, 1983, pp. 363-368. Clinical experience has proven that mirrors used to expand the patient's visual field awareness cause significant disorientation and perceptual confusion due to the reversal of images. The use of reversed telescopes is theoretically an approach that will maximize visual field awareness, but experience has shown that utilization of reversed telescopes is not effective in the treatment of hemianopic visual field loss. Furthermore, the mirror and reversed telescope techniques suffer from the fact that they are cosmetically unacceptable to patients.
The use of prisms to treat visual field loss has been previously described by Wayne W. Hoeft, 0.D. in Low Vision (ed. Eleanor E. Faye; Charles C. Thomas, 1973, pp. 103-113) which shows the use of large prisms placed in front of the lens of a pair of glasses. The use of one prism for each lens is demonstrated in this article. The cosmetic appearance of such a system was cited as a serious problem in patient acceptance of the system.
One proposed resolution to the cosmetic problems of the use of prisms is the use of Fresnel thin wafer prisms, the use of which are described in detail by Randall T. Jose, 0.D. and Audrey J. Smith ("Increasing Peripheral Field Awareness with Fresnel Prisms," Optical Journal and Review of Optometry, Vol. 113, No. 12, Dec. 15, 1976, pp. 33-37). This article discloses the use of one Fresnel prism placed on the lens.
Our clinical experiences have shown us that the utilization of a Fresnel press-on prism is an ineffective final rehabilitative aid in the management of a patient suffering visual field loss. Patients find these prisms to cause significant loss of contrast due to the poor optical quality as well as the prismatic distortion, all resulting in reduced resolution. Patients also complain of the poor cosmetic appearance and the significant discoloration evident over a short period of time. They often find the Fresnel press-on prism to be a nuisance rather than a help.
For the purposes of this application, "prism" shall be understood to mean a wedge prism (or a portion thereof), specifically excluding Fresnel prism. The base of the prism is its widest portion, or more precisely, the edge of the prism in the direction in which the prism bends or refracts light. On the other hand, the apex of the prism is its narrowest portion.